This invention relates to the casting of steel strip. It has particular application for continuous casting of thin steel strip less than 5 mm in thickness in a roll caster.
In a roll caster, molten metal is cooled on casting surfaces of at least one casting roll and formed into thin cast strip. In roll casting with a twin roll caster, molten metal is introduced between a pair of counter rotated casting rolls that are cooled. Solidified metal shells are formed on the moving casting surfaces and are brought together at a nip between the casting rolls to produce thin cast strip delivered downwardly from the nip. The term “nip” is used herein to refer to the general region in which the casting rolls are closest together. In any case, the molten metal is usually poured from a ladle into a smaller vessel, and from there, flows through a metal delivery system to distributive nozzles located generally above the nip between the casting rolls. During casting, the molten metal is delivered between the casting rolls to form a casting pool of molten metal supported on the casting surfaces of the rolls adjacent the nip and along the length of the casting rolls. Such casting pool is usually confined between side plates or dams, which are held in sliding engagement adjacent the ends of the casting rolls, so as to confine the two ends of the casting pool.
When casting thin steel strip with a twin roll caster, the molten metal in the casting pool will generally be at a temperature of the order of 1500° C. and above. It is therefore necessary to achieve very high cooling rates over the casting surfaces of the casting rolls. A high heat flux and extensive nucleation on initial solidification of the metal shells on the casting surfaces is needed to form the steel strip. U.S. Pat. No. 5,760,336, incorporated herein by reference, describes how the heat flux on initial solidification can be increased by adjusting the steel melt chemistry such that a substantial portion of the metal oxides formed are liquid at the initial solidification temperature, and in turn, a substantially liquid layer is formed at the interface between the molten metal and casting surfaces. As disclosed in U.S. Pat. Nos. 5,934,359 and 6,059,014 and International Application AU 99/00641, the disclosures of which are incorporated herein by reference, nucleation of the steel on initial solidification can be influenced by the texture of the casting surface. In particular, International Application AU 99/00641 discloses that a random texture of discrete protrusions formed in the casting surfaces can enhance initial solidification by providing substantial nucleation sites distributed over the casting surfaces.
Attention has been given in the past to the steel chemistry of the melt, particularly in the ladle metallurgy furnace before thin strip casting. We have given attention in the past to the oxide inclusions and the oxygen levels in the steel metal, and their impact on the quality of the steel strip produced. We have also found that the quality of the steel strip and the production of the thin steel strip is also enhanced by control of the hydrogen levels and nitrogen levels in the molten steel. Controlling hydrogen and nitrogen levels has in the past been the subject of investigation in slab casting, but to our knowledge has not been a focus of attention in thin strip casting until our work. For example, see Control of Heat Removal in the Continuous Casting Mould, by P. Zasowski and D. Sosinsky, 1990 Steelmaking Conference Proceedings, 253-259; and Determination and Prediction of Water Vapor Solubilities in CaO—MgO—SiO2 Slags, by D. Sosinsky, M. Maeda and A. Mclean, Metallurgical Transactions, vol. 16b, 61-66 (March 1985).
We have also described in parent U.S. patent application Ser. No. 10/961,300, filed Oct. 8, 2004, and U.S. provisional patent application 60/510,479, filed Oct. 10, 2003, that controlling the hydrogen level to below about 6.9 ppm and the nitrogen level to below about 120 ppm, while maintaining the sum of the partial pressures of hydrogen and nitrogen to not more than 1.15 atmospheres is a substantial advance in thin strip casting. We described that by controlling the hydrogen and nitrogen levels in plain carbon steel strip unique composition and production qualities can be produced by roll casting.
Now, we have found that the amount of heat flux from the metal shells is sensitive to the nitrogen levels in the molten steel in the casting pool, and therefore, by controlling the nitrogen levels in the casting pool the heat flux between the molten metal and the casting rolls can be controlled. Disclosed is a method of casting steel strip comprising the steps of:                determining a desired heat flux set point in casting thin cast strip from molten metal from a casting pool between casting rolls in a twin roll caster;        calculating the heat flux during casting of thin steel strip from molten metal in the casting pool in the twin roll caster; and        changing the nitrogen concentration in the casting pool to adjust the heat flux to the desired heat flux set point.        
The nitrogen concentration in the casting pool may be controlled during the casting campaign to adjust for changes roughness of the casting rolls and other operating conditions to maintain the calculated heat flux to the desired heat flux set point or some adjustment therefrom as desired. The controlling step may be done manually by an operator or done automatically by a proportional controller.
Also disclosed is a method of casting thin steel strip comprising the steps of:                introducing molten plain carbon steel on casting surfaces of at least one casting roll with the molten steel having a nitrogen content below about 120 ppm and a hydrogen content below about 6.9 ppm and such that the sum of partial pressure of nitrogen and partial pressure of hydrogen is no more than 1.15 atmospheres;        forming a casting pool of molten metal on the casting surfaces of the casting roll;        causing the nitrogen level in the molten metal in the casting pool to vary depending on casting speed and strip thickness to control heat flux between the casting pool and the surfaces of the casting roll to a desired value; and        solidifying the molten steel to form metal shells on the casting rolls having nitrogen and hydrogen levels reflected by the content thereof in the molten steel to form thin steel strip.        
The method of casting steel strip nay be carried out by the steps comprising the following:                assembling a pair of cooled casting rolls having a nip between them and confining end closures adjacent to ends of the casting rolls;        introducing molten plain carbon steel between the pair of casting rolls to form a casting pool on the casting rolls with the end closures confining the pool, with the molten steel having a nitrogen content below about 120 ppm and a hydrogen content below about 6.9 ppm and such that the sum of partial pressure of nitrogen and partial pressure of hydrogen is no more than 1.15 atmospheres;        causing the nitrogen level in the molten metal in the casting pool to be varied depending on casting speed and strip thickness to control heat flux between the casting pool and the surfaces of the casting roll to a desired value;        counter-rotating the casting rolls and solidifying the molten steel to form metal shells on casting surfaces of the casting rolls having nitrogen and hydrogen levels reflected by the content of the molten steel to provide for the formation of thin steel strip; and        forming solidified thin steel strip through the nip between the casting rolls to produce a solidified steel strip delivered downwardly from the nip.        
In any of these methods, nitrogen level in the molten melt in the casting pool may be controlled by introducing the nitrogen in the metal delivery system, e.g. in the tundish, above the casting pool. In any of these methods, the nitrogen levels in the casting pool may be maintained below 100 ppm or 85 ppm. In any of these methods, the hydrogen content may be between 1.0 and 6.5 ppm.
In any of these methods, the heat flux can be indirectly correlated with the sum of the calculated partial pressures of nitrogen and hydrogen in the casting pool. The sum of the partial pressures of nitrogen and hydrogen calculated from the concentrations of nitrogen and hydrogen in the molten metal of the casting pool may be no more than 1.15 atmospheres, or may be between 0.1 and 0.8 atmospheres. Note that the partial pressures of nitrogen and of hydrogen are calculated from the levels, or concentrations, of nitrogen and hydrogen in the casting pool. See Richard J. Fruehan, ed., The Making, Shaping and Treating of Steel § 2.4.2.1 Table 2.9 (11th ed. 1998). As a practical matter, the nitrogen and hydrogen concentrations are typically measured in the tundish adjacent the casting pool, and it is the measured levels of nitrogen and hydrogen in the tundish that are reported herein. There is typically a slight nitrogen pick up of the molten metal between the tundish and the casting pool; however, the concentrations of the nitrogen and hydrogen in the casting pool are believed to be the concentrations related to control of the heat flux.
The method of casting steel strip may include the steps of correlating the heat flux with the sum of the calculated partial pressures of hydrogen and nitrogen associated the nitrogen and hydrogen concentrations in the casting pool, measuring the hydrogen and nitrogen concentrations in the molten metal going into the casting pool during casting, calculating the sum of the partial pressures associated with the measured hydrogen and nitrogen concentrations, and changing the concentration of nitrogen in the casting pool to provide the sum of the calculated partial pressures associated with the hydrogen and nitrogen concentrations correlated with the desired heat flux.
Plain carbon steel for purpose of the present invention is defined as less than 0.65% carbon, less than 2.5% silicon, less than 0.5% chromium, less than 2.0% manganese, less than 0.5% nickel, less than 0.25% molybdenum and less than 0.05% aluminum, together with of other elements such as sulfur, oxygen and phosphorus which normally occur in making carbon steel by electric arc furnace. Low carbon steel may be used in these methods having a carbon content in the range 0.001% to 0.1% by weight, a manganese content in the range 0.01% to 2.0% by weight, and a silicon content in the range 0.01% to 2.5% by weight. The steel may have an aluminum content of the order of 0.05% or less by weight. The aluminum may, for example, be as little as 0.008% or less by weight. The molten steel may be a silicon/manganese killed steel.
In these methods, the sulfur content of the steel maybe 0.01% or less. For example, the sulfur content of the steel may be 0.007% by weight.
The nitrogen and hydrogen levels are the dissolved nitrogen and hydrogen in the molten metal, and not nitrogen and hydrogen combined with other elements in compounds in the molten metal. In these methods, the nitrogen may be measured by optical emission spectrometry, calibrated against the thermal conductivity method as described below. The hydrogen levels may be determined by a Hydrogen Direct Reading Immersed System (“Hydris”) unit, made by Hereaus Electronite.
The maximum allowable nitrogen and hydrogen levels in the casting pool may be associated with the sum of the calculated partial pressures of hydrogen and nitrogen and may be such as to not exceed 1.15 or 1.0 atmospheres. Higher pressures may be utilized in certain conditions, and the associated levels of nitrogen and hydrogen can be corresponding higher. For example, as explained below, a ferrostatic head may be 1.15, causing the nitrogen levels and hydrogen levels to be higher in the casting pool. But for purposes of the parameters of the present methods, the partial pressures of nitrogen and hydrogen are calculated from the measured levels of nitrogen and hydrogen in the molten metal using the equation described below. The calculated partial pressure would be the gas pressure that the nitrogen or hydrogen exerts in equilibrium with the liquid steel; the partial pressures of nitrogen and hydrogen in the casting pool are during a casting campaign usually determined by the gases injected into the chamber in which the casting pool is formed.
The present invention provides cast steel strip with unique properties that are described by the methods by which it is made. This steel strip may be described as plain carbon steel.